Detection system for chemical-mechanical planarization tool

ABSTRACT

Methods and apparatus are provided for endpoint detection in a chemical mechanical planarization (CMP) process. Reflectance spectra data is taken periodically in different areas of a surface of a semiconductor wafer during a chemical mechanical planarization process. Three different reflectance spectra are identified to determine a status of the CMP process. A first reflectance spectra data corresponds to light reflected predominately from a layer of material on the surface of the semiconductor wafer. A second reflectance spectra corresponds to the layer of material being thinned such that the second reflectance spectra is modified by an underlying layer of material. A third reflectance spectra corresponds to light reflected predominately from the underlying layer of material.

TECHNICAL FIELD

The present invention generally relates to the chemical mechanicalplanarization of semiconductor wafers, and more particularly relates tothe detection of an end point of a chemical mechanical planarizationprocess.

BACKGROUND

A flat disk or “wafer” of single crystal silicon is the basic substratematerial in the semiconductor industry for the manufacture of integratedcircuits. Semiconductor wafers are typically created by growing anelongated cylinder or boule of single crystal silicon and then slicingindividual wafers from the cylinder. The slicing causes both faces ofthe wafer to be extremely rough. The front face of the wafer on whichintegrated circuitry is to be constructed must be extremely flat inorder to facilitate reliable semiconductor junctions with subsequentlayers of material applied to the wafer. Also, the material layers(deposited thin film layers usually made of metals for conductors oroxides for insulators) applied to the wafer while building interconnectsfor the integrated circuitry must also be made a uniform thickness.

Integrated circuits manufactured today are made up of literally millionsof active devices such as transistors and capacitors formed in asemiconductor substrate. Integrated circuits rely upon an elaboratesystem of metallization in order to connect the active devices intofunctional circuits. A typical multilevel interconnect 100 is shown inFIG. 1. Active devices such as MOS transistors 107 are formed in and ona silicon substrate or well 102. An interlayer dielectric (ILD) 104,such as SiO₂ is formed over silicon substrate 102. ILD 104 is used toelectrically isolate a first level of metallization that is typicallyaluminum (Al), with copper (Cu) increasing in popularity, from theactive devices formed in substrate 102 to interconnections 108 of thefirst level of metallization. In a similar manner metal vias 112electrically couple interconnections 114 of a second level ofmetallization to interconnections 108 of the first level ofmetallization Contacts 106 and vias 112 typically comprise a metal 116such as tungsten (W) surrounded by a barrier metal 118 such astitanium-nitride (TiN). Additional ILD/contact and metallization layersmay be stacked one upon the other to achieve the desiredinterconnections. The ILD/contact and metallization layers may beformed, for example, using a dual damascene process.

Planarization is the process of removing projections and otherimperfections to create a flat planar surface, both locally andglobally, and/or the removal of material to create a uniform thicknessfor a deposited thin film layer on a wafer. Semiconductor wafers areplanarized or polished to achieve substantially smooth, flat finishbefore performing process steps that create the integrated circuitry orinterconnects on the wafer. A considerable amount of effort in themanufacturing of modern complex, high density multilevel interconnectsis denoted to the planarization of the individual layers of theinterconnect structure. Nonplanar surfaces create poor opticalresolution of subsequent photolithography processing steps. Poor opticalresolution prohibits the printing of high-density lines. Another problemwith nonplanar surface topography is the step coverage of subsequentmetallization layers. If a step height is too large there is a seriousdanger that open circuits will be created. To this end,chemical-mechanical planarization (CMP) tools have been developed toprovide controlled planarization of both structured and unstructuredwafers.

CMP consists of a chemical process and a mechanical process actingtogether, for example, to reduce height variations across dielectricregion, clear metal deposits in damascene processes or remove excessoxide in shallow trench isolation fabrication. The chemical-mechanicalprocess is achieved with a liquid medium containing chemicals that reactwith the front surface of the wafer when it is mechanically stressedduring the planarization process.

In a conventional CMP tool for planarizing a wafer, a wafer is securedin a carrier connected to a shaft. The shaft is typically connected tomechanical means for transporting the wafer between a load or unloadstation and a position adjacent to a polishing pad mounted to a rigid orflexible platen or supporting surface. Pressure is exerted on the backsurface of the wafer by the carrier in order to press the front surfaceof the wafer against the polishing pad, usually in the presence ofslurry. The wafer and/or polishing pad are then moved in relation toeach other via motor(s) connected to the shaft and/or supporting surfacein order to remove material in a planar manner from the front surface ofthe wafer.

It is often desirable to monitor the front surface of the wafer duringthe planarization process. One known method is to use an optical systemthat monitors the front surface of the wafer in situ by positioning anoptical probe under the polishing pad. Laser interferometry, signaltemplate matching and multifrequency analysis techniques, as well asothers, are known monitoring methods. The signal from the probe may betransmitted and received through an opening in the polishing pad. Theopening in the polishing pad may be filled with an optically transparentmaterial, or “window”, in order to prevent polishing slurry or othercontaminants from being deposited into the probe and obscuring theoptical path to the wafer. The data from the optical system is typicallyanalyzed by a control system to determine the current condition of thefront surface of the wafer. It is possible to terminate theplanarization process (call end-point) once the front surface of thewafer has reached a desired condition. An optical system may be used tocompensate for drifts in the planarization process, variability in theassociated consumables (polishing pads and slurries), and variability inthe thickness of incoming wafers.

A reliable end-point detection system is critical for maintaining theoptimum CMP process. The end-point system detects the point in theplanarization process when the overburden being polished is removedeverywhere across the wafer. Excessive removal of overburden from thefront surface of the wafer, whether a raw sheet film, or an STI, metalor dielectric layer structure on the front wafer surface, may damage thewafer.

Accordingly, it is desirable to monitor a surface of a wafer during aplanarization process and have a methodology for accurate detection. Inaddition, it is desirable to take measurements frequently and having asmall spot size for higher sensitivity in detecting small residuals. Itis also beneficial to be able to identify when the material beingremoved approaches a clearing stage to prepare for ending the CMPprocess. Furthermore, other desirable features and characteristics ofthe present invention will become apparent from the subsequent detaileddescription and the appended claims, taken in conjunction with theaccompanying drawings and the foregoing technical field and background.

BRIEF SUMMARY

A method is provided for endpoint detection in a chemical mechanicalplanarization (CMP) process. Reflectance spectra data is takenperiodically in different areas of a surface of a semiconductor waferduring a chemical mechanical planarization process. Three differentreflectance spectra are identified to determine a status of the CMPprocess. A first reflectance spectra data corresponds to light reflectedpredominately from a layer of material on the surface of thesemiconductor wafer. A second reflectance spectra corresponds to thelayer of material being thinned such that the second reflectance spectrais modified by an underlying layer of material. A third reflectancespectra corresponds to light reflected predominately from the underlyinglayer of material.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will hereinafter be described in conjunction withthe following drawing figures, wherein like numerals denote likeelements, and

FIG. 1 is a greatly expanded cross section view of a semiconductor chip;

FIG. 2 is a greatly expanded cross section view of an interconnect in asemiconductor chip;

FIG. 3 is a simplified cross section view of an apparatus used topractice the present invention;

FIG. 4 is a reflectance spectra of a barrier layer such as Ta or TaN atthe start of a CMP process;

FIG. 5 is a reflectance spectra of the barrier layer being cleared awayexposing a dielectric layer such as silicon dioxide;

FIG. 6 is an intermediate reflectance spectra that occurs as a barrierlayer is thinned in accordance with the present invention;

FIG. 7 is a normalized reflectance spectra for the case when themeasurement produces a reflectance spectra that is predominately fromlight reflected from the targeted material;

FIG. 8 is a normalized reflectance spectra for the case when the barrierlayer is thinned during the CMP process and the reflected light ismodified by the underlying layer of material;

FIG. 9 is a normalized reflectance spectra for the case when the barrierlayer is removed and the reflected light is predominately from theunderlying dielectric layer; and

FIG. 10 is a flow diagram of an exemplary process in accordance with thepresent invention.

DETAILED DESCRIPTION

The following detailed description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. Furthermore, there is no intention to be bound by anyexpressed or implied theory presented in the preceding technical field,background, brief summary or the following detailed description.

CMP of copper will become one of the most common and critical chemicalmechanical planarization processes when the copper interconnecttechnology starts to dominate the fabrication of integrated circuits.FIG. 2 illustrates some of the potential problems if excessiveoverburden is removed, in this case a barrier layer 203, from the frontsurface of a wafer. At time T1 a layer of deposited copper 200 remainson the wafer. The copper layer is removed with a CMP step exposingbarrier layer 203. In general, barrier layer 203 is deposited beforecopper 200. Barrier layer 203 forms a layer on the bottom and sidewallsof a cavity, via opening or trench. Barrier layer 203 is also on thewafer surface underlying copper 200. For example, Tantalum (Ta) orTantalum Nitride (TaN) is often used to form barrier layer 203. Barrierlayer 203 typically has good adhesive qualities to silicon dioxide 201and copper 200.

A second CMP step removes barrier layer 203 on the surface of the wafer.Ideally, the CMP step should terminate at time T2 or just slightlythereafter for an optimum planarized surface. However, if theplanarization process is not terminated quickly enough, excessiveremoval of copper in the interconnects 202 may occur as shown at timeT3. The dishing of the copper interconnects 202 occurs because thecopper is softer than silicon dioxide 201 and is therefore removed at afaster rate.

End-point detection and monitoring is required for barrier layer 203during a CMP step due to the variations in the incoming thicknessdistribution of barrier layer 203 as well as microstructural variationsin the deposited barrier film. This may result in nonuniform clearing ofbarrier layer 203 across the front surface of the wafer. Severalproblems exist with conventional in situ monitoring techniques thatlimit their ability to accurately detect the clearing of barrier layer203. In particular, it is very difficult to detect an end point whencomparing reflectance spectra of barrier layer 203 and reflectancespectra of a dielectric layer such as silicone dioxide 201.

Some conventional monitoring systems use a laser interferometerpositioned below a rotating polishing pad. However, this type ofmonitoring system can only take measurements while the laser and opticalpath through the polishing pad are in alignment with the front surfaceof the wafer. This situation creates a very narrow time-period duringeach rotation of the polishing pad that measurements can be taken. Thenarrow time-period during each rotation of the polishing pad thatmeasurements can be taken. The narrow time-period and the delay betweentime-periods for taking measurements created by the rotating polishingpad greatly diminish the capabilities of the monitoring system.

In addition, some conventional systems tend to measure a relativelylarge spot, or integrate a number of large spot, or integrate a numberof large spots (smear measurement), on the wafer's surface to increasethe surface area of the wafer being monitored. However, thesemeasurements create a system with very poor sensitivity that cannotdistinguish high metallization density from mere residual metal.Improved sensitivity is required to detect residuals on the wafer'ssurface that are on the order of the spot size, but may influence thequality of the planarization process.

An apparatus for practicing the present invention will now be discussedwith reference to FIG. 3. During a planarization process, a wafer 100may be transported by a carrier 301 to a position adjacent andsubstantially parallel to a working surface or polishing pad 309. Thefront surface of the wafer 100 is pressed against the polishing pad 309fixed to a supporting surface 211, preferably in the presence of aslurry (not shown). The front surface of the wafer 100 is planarized begenerating relative motion between the front surface of the wafer 100and the polishing pad 309 thereby removing material from the frontsurface of the wafer 100.

The apparatus includes a plurality of probes 305 a-c positioned beneaththe polishing pad 309 to transmit light to, and receive reflected lightfrom, the front surface of the wafer 100. Three probes 305 a-c areillustrated in FIG. 3, however, any number of probes may be used. Thegreater the number of probes, the faster a complete scan of the wafermay generally be taken, but each additional probe increases the expenseand complexity of the system. The probes 305 a-c are preferablybifurcated to allow separate optical paths for the transmitted andreflected light to, and receive reflected light from, a particularannular band on the front surface of the wafer 100. If an orbital CMPtool is used having a relatively small orbital radius, each probe 305a-c may be used to monitor a single annular band. The annular bands insuch an orbital CMP tool may be made to overlap to ensure the entirefront surface of the wafer 100 may be altered by a multizone carrier301.

The carrier 301 is preferably rotated about its central axis as itpresses the front surface of the wafer 100 against the polishing pad 309during the planarization process. The rotational speed of the carrier301 is preferably selected to optimize the planarization process. Theoptimum rotational speed for the planarization process may be determinedthrough computer models or by empirical means. Rotational speed of about12 rpm for the carrier 301 have been found to produce satisfactoryplanarization results while permitting the transmittance and receptionof reflected light from the front surface of the wafer 100. The carrier301 may also be moved along the polishing pad 309 to enhance theplanarization process of the wafer 100.

The carrier 301 may be adapted to permit biasing the pressure exerted ondifferent areas of the back surface of the wafer 100. Areas of the backsurface of the wafer 100 receiving a higher (or lower) pressure willtypically increase (or decrease) the removal rate of material fromcorresponding areas on the front surface of the wafer 100. Removal ratesof material from planarization processes are typically substantiallyuniform within concentric annular bands about the center of the wafer,but often differ greatly from band to band. To correct for this commonproblem, the carrier 301 is preferably capable of exerting differentpressures in a plurality of different areas while maintaining a uniformpressure within each area. Since removal rates for planarizationprocesses tend to be uniform within concentric bands on the frontsurface of the wafer 100, the carrier 301 is ideally able to apply auniform pressure over each concentric band o the back surface of thewafer 100. In addition, since removal rates tend to differ from band toband on the front surface of the wafer 100, the carrier 301 is alsoideally able to apply different pressures over different bands on theback surface of the wafer 100. Examples of such carriers are disclosedin U.S. Pat. No. 5,882,243; U.S. Pat. No. 5,916,016; U.S. Pat. No.5,941,758; and U.S. Pat. No. 5,964,653 and are hereby incorporated byreference. The greater the number of concentric annular bands, thegreater the process flexibility in adjusting the carrier 301 to theproblems encountered in the planarization process. However, thecomplexity and cost of the carrier also increases as the number ofadjustable bands increases. A carrier with three (3) adjustableconcentric pressure bands is expected to give you improved processflexibility while keeping the complexity of the carrier to a manageablelevel. Since the need for improved process results is almost certainlygoing to increase in the future, the preferred number of controllablebands within the carrier will also likely increase in the future.

A simplified view of one possible multizone carrier 301 is illustratedin FIG. 3. Carrier 301 has three concentric plenums: a central 303 a,intermediate 303 b and peripheral 303 c plenum. A flexible membrane 314provides a surface for supporting the wafer 100 while an inner 315 andan outer 316 ring provides barriers for separating the plenums 303 a-c.The pressure within the central 303 a, intermediate 303 b and peripheral303 c plenums may be individually communicated through passageways 304a-c by respective controllable pressure regulators 313 a-c connected toa pump 312. A rotary union 302 may be used in communicating the pressurefrom the pump 312 and pressure regulators 313 a-c to their respectiveplenums 303 a-c if the carrier 301 is rotated. Thus, each concentricplenum 303 a-c may be individually pressurized to create threeconcentric bands to press against the back surface of the wafer 100.Each plenum 303 a-c may therefore have a different pressure, but eachconcentric band will therefore have a uniform pressure within the bandto press against the back surface of the wafer 100.

A supporting surface 211 maybe used to support the abrasive surface orpolishing pad 309. The supporting surface 211 may be a rigidsubstantially planar surface comprising aluminum, stainless steal,ceramic, titanium, polymer or other such rigid, non-corrosive material.Alternatively, the supporting surface 211 for a polishing pad 309. Aslurry delivery system (not shown) is preferably incorporated into thesupporting surface 211 for delivery of slurry onto the polishing pad309.

The supporting surface 211 may be connected to a motion generator 500for creating relative motion between the front surface of the wafer 100and the polishing pad 309. Various motions for the supporting surface211 are already known. For example, U.S. Pat. No. 5,498,196 shows anexample of a rotational CMP tool; U.S. Pat. No. 5,692,947 shows anexample of a linear belt system; U.S. Pat. No. 5,707,274 shows anexample of a rotary drum system; and U.S. Pat. No. 5,554,064 shows anexample of an orbital tool, all of which are hereby incorporated byreference.

A multiprobe metrology instrument, e.g. a multiprobe end-point detectionsystem 308, may be used to analyze data from the front surface of awafer 100. Multiple probes allow samples to be taken at a desireddensity across the face of the wafer 100 in a shorter time than a singleprobe system, but increase the complexity of the system. This isaccomplished since additional probes prevent or shorten the time whenthere are no probes under the front surface of the wafer 100 and mayeven allow multiple points to be sampled substantially simultaneously.It is highly desirable to take samples at a desired spatial densityacross the entire surface of the wafer 100 (a “scan”) as quickly aspossible to obtain the best possible data to analyze. The surface of thewafer 100 changes rapidly during the planarization process and a longinterval between samples will result in the early measurement notaccurately reflecting the condition of the wafer 100 when the latermeasurements are taken. Interpolation, extrapolation or modelingsoftware may be used to make estimates that compensate for temporalvariations in samples, but the preferred method is to complete the scanas quickly as possible.

As a specific example, a short scan time may help avoid dishing orerosion in a barrier layer chemical mechanical planarization processused in forming interconnect. Once an area has been cleared of thebarrier material, a scan of the entire wafer is preferably completedwith the desired resolution within the time necessary to preventexcessive erosion or dishing of features in the cleaned areaMeasurements are most important once an area has cleared to make surethe planarization process stops before that area or any other areaexperiences erosion or over-polishing.

The multiprobe end-point detection system 308 may be used to determineareas on the front surface of the wafer 100 that need an increase ordecrease in material removal rate. The areas that need an increase ordecrease in material removal rate will typically take the form ofconcentric rings about an annular central region on the front surface ofwafer 100.

A multiprobe end-point detection system 308 is advantageous in CMP toolswhere a wafer does not remain over the same area of a polishing pad, asin a conventional rotational CMP tool. Multiple probes may be used toreduce the amount of time when no probe is under the wafer or may beused to increase the number of points sampled when more than one probeis under the wafer. A multiprobe end-point detection system 308 is alsoadvantageous in systems where the wafer remains substantially over thesame are of a polishing pad, as in a conventional orbital system. Asillustrated in FIG. 3, the probes 306 a-c may be positioned where theyare always, or almost always, under the wafer 100 thereby allowingmultiple probes 306 a-c greatly reduces the time necessary to complete ascan and greatly increases the accuracy of the analysis of the frontsurface of the wafer 100 by limiting temporal difference in the samples.

Referring still to FIG. 3, an emitter or flash lamp 317 may be used toinitiate a light signal to travel through one or more fiber opticalcables 307 a-c. One or more probes 306 a-c are preferably positioned asclose as possible to a transparent area 305 a-c to enhance the opticalcommunication. The reflected light signal from the wafer 100 may becaptured by a probe 306 a-c and routed to a metrology instrument 318,such as spectrometer, via fiber optic cables 307 a-c. The invention maybe practiced with a variety of probes, flash lamps and fiber opticalcables that are known in the art.

While measurement averaging or integration over a large area may be usedto collect samples, a flash lamp 317 allows high-speed discretemeasurements to be taken. Discrete measurements provide finer spatialresolution and are capable of detecting smaller residuals on the frontsurface of the wafer 100. The light signal is preferably a broad bandspectrum of light so that the intensity of the reflected light signalmay be analyzed at multiple wavelengths. In one embodiment, the spectrumpreferably includes light between 300 and 800 nm in wavelength. As aspecific example, a Xe flash lamp 317 may be used to generate the lightsignal. Although a pulsed light system is preferred, using a non-pulsedlight system is also a viable alternative.

The flash duration of the flash lamp 317 should be as short as possibleto minimize the amount of relative motion between the surface of thewafer and the flash lamp 317 and probe 306 a-c during signal collection.Relative motion between the surface of the wafer 100 and the probe willcreate a smear effect and decrease the sensitivity of the measurement ifthe illumination were to endure over the relatively large duty cycleperiod of a grating spectrometer. The flash duration does need to belong and intense enough, however, to provide enough signal intensity forthe probes 306 a-c to collect the reflected light from the surface ofthe wafer 100. The flash duration is preferably less than about tenmicroseconds.

The flash is optimally repeated as quickly as possible in order togather the greatest amount of sample data However, two factors limit theusefulness of extremely fast sampling rates. The first is that eachflash provides a tremendous amount of data that must be quicklyanalyzed. Data that has been gathered, but that cannot be timelyanalyzed does not benefit the system The second factor is that some timemust be allowed to pass between measurement in order to relative motionbetween the front surface of the wafer 100 and the probes 306 a-c tomove the measurement location. The measurements are preferably evenlydistributed, and as close as possible, across the front surface of thewafer 100.

The spot size of light from the flash lamp 317 is preferably slightlylarger than the largest feature that is supposed to remain on thesurface of the wafer 100. This will prevent a fully planarized area fromgiving a false reading indicating that residuals remain. This couldhappen if a measurement were taken over a large feature with a spot sizesmaller than the large feature. On the other hand, a spot size that istoo big may miss residuals that are smaller than the spot size. Theoptimum spot size is larger than the largest feature while also beingsmaller than the smallest residual it is required to detect. As featuresizes continue to decrease and the requirements for semiconductormanufacturing continue to become more stringent, the optimum spot sizewill decrease. A spot size of one to three mm in diameter acceptable forthe most current semiconductor manufacturing requirements with smallerspot sizes likely required in the future.

There are preferably enough probes properly positioned in the CMP toolto allow sampling across the entire front surface of the wafer 100during the planarization process. Typical orbital CMP tools, due to thesmall relative movements between the front surface of the wafer 100 andthe polishing pad 309, need multiple probes that preferably have aslight overlap of coverage to insure all areas on the front surface ofthe wafer 100 are sampled. Each probe in a conventional orbital CMPtool, with a rotating carrier 301, will examine an annular band on thefront surface of the wafer 100 approximately the width of the diameterof the orbit. Thus, all the data for a particular annular band on thefront surface of the wafer 100 in a conventional orbital CMP tool comesfrom a single probe thereby simplifying the analysis of the data.

The metrology instrument 318, preferably a grating spectrometer(s),accepts the incoming reflected light signal and converts the lightsignal into data indicating the intensity of the reflected light aplurality of different wavelengths. The data may then be transmitted toa control system 311 for analysis. The control system 311 is able todetermine the condition of the front surface of the wafer 100 from thedata A number of numerical methods may be used to determine when theplanarization process should be terminated, i.e. end-point called. Forexample, end-point may be called after a predetermined over-polish timehas occurred starting from the time a predetermined percentage ofmeasurements show an absence of a barrier material. The over-polish timeensures a complete clearing of the barrier material. The over-polishtime and the percentage of measurements showing an absence of barriermaterial are preferably determined empirically due to variations fromplanarization process to planarization process.

Another method of analyzing the data compares the clearing time fordifferent concentric areas on the front surface of the wafer 100. Thismethod may be simplified when each probe monitors a particular, possiblyoverlapping, concentric band as would be the case when used with orbitalCMP tools with a relatively small orbital radius. For example, probes306 a-c below areas that clear first indicate bands that are beingpolished too quickly in comparison to other bands. Corrective action maythen be taken for that wafer 100 or the information may be used toimprove the planarization process for incoming wafers.

The control system 311 may make immediate adjustments to theplanarization process based on the analysis of the measurements. Forexample, increasing or decreasing the pressure on the back surface ofthe wafer 100 during the planarization process has been found toincrease or decrease, respectively, the removal rate at the periphery ofthe wafer 100 with respect to the center of the wafer 100. As anotherexample, more or less slurry may be distributed near areas that havebeen found to need increased or decreased, respectively, removal rates.As yet another example, the rotation speed of the carrier 301 may beincreased or decreased, respectively, removal rates. As yet anotherexample, the rotation speed of the carrier 301 may be increased ordecreased to increase or decrease, respectively, the removal rate at theperiphery of the wafer 100. However, the preferred method is to use amultizone carrier 301 to alter the removal rate at different areas ofthe front surface of the wafer 100. Specifically, the pressure may beincreased or decreased in zones over areas that need an increase ordecrease in material removal rate, respectively, on the front surface ofthe wafer 100. In addition, the results from planarized wafers 100 maybe used to change the process parameters for incoming wafers. Thisallows process drift within the planarization process to be detected andcompensated for as it happens.

To determine the condition of the front surface of the wafer 100, thelocation for each measurement should be known. One possible method is totrack only the radial position for each measurement and take at leastone measurement at various radial positions in find enough increments toprovide a desired sampling resolution. This method assumes that eachmeasurement accurately represents the condition of the wafer 100 atevery point having the same radial position. Since wafers 100 generallyhave bands that planarize at approximately the same rate, this methodprovides a simple approximation of the condition of the front surface ofthe wafer 100. In this manner, measurements may be taken across thefront surface of the wafer 100 at a desired spatial resolution thatprevents a problem area larger than the desired resolution from goingunobserved.

Various devices may be used to track the location of the measurement onthe front surface of the wafer 100. For example, an encoder 320 may beused to track the position of the carrier 301 (and thus the wafer) andtransmit this information via communication line 319 to the controlsystem 311. In a similar manner, an encoder 321 may be used to track theposition of the supporting surface 211 (and thus the probes) andtransmit this information via communication line 322 to the controlsystem 311. The wafer 100 may need to be firmly held in the carrier 301to prevent the wafer 100 from spinning and randomizing its orientation.For example, the wafer 100 may be held in place by applying suction tothe back surface of the wafer 100 through the membrane 314 or bycreating a tacky bottom surface for the membrane 314. The control system311 thus has the information necessary to match the data from themetrology instrument 318, preferably a spectrometer, with the data'scorresponding location on the front surface of the wafer 100.Alternatively, modeling software for the mechanical mechanisms thatcause the relative motion between the wafer 100 and the polishing pad308 may also be used to predict the location of each measurement on thefront surface of the wafer 100. Modeling software is also useful indetermining desirable motions for the carrier 301 and supporting surface211, and thus the wafer 100 and probes, that will produce a pattern ofmeasurements as evenly distributed as possible. Small adjustments to thedesired relative motion between the wafer 100 and the polishing pad 309may be made to improve the distribution of measurements while havingonly a minimum impact on the planarization process. An evenlydistributed pattern may shorten the time of a scan by requiring theminimum number of measurements and the least amount of data processing.However, the measurement locations do not have to be evenly distributed,but the largest space between measurements is preferably smaller thanthe targeted residual detection capability.

Alternatively, the measurements may be analyzed until the largestpossible remaining residual is of a predetermined size. Once all theremaining residuals are of the predetermined size or smaller, the wafer100 may be planarization for an additional time (over-polish time) toremove the remaining residuals. The additional planarization time may befound by empirically determining the maximum amount of time necessary toplanarize away residuals of the predetermined size.

All of the described techniques help to control the rate and uniformityof material removal with the proper input. The problem for some chemicalmechanical planarization processes is the determination of the exactstatus at any point in time. For example, in many CMP processes thetransition from having a material present on the wafer surface to thematerial being cleared away will give many indicators over time thatallow precise determinations to be made. For other CMP processes it isvery difficult to tell when the material is cleared away.

One such chemical mechanical planarization process is removing a barrierlayer having an underlying dielectric layer. FIG. 4 is a reflectancespectra 400 of a barrier layer such as Ta or TaN at the start of a CMPprocess. The Y-axis is intensity and the X-axis is the light wavelength.As shown, a broadband spectrum of light is used having a wavelengthranging from approximately 200 nanometers to 800 nanometers. Threedifferent reflectance spectra are shown because three probes were usedto take the measurements. Each probe took measurements at differentlocations on the wafer. Notice that peaking occurred at wavelengthsslightly less than 500 nanometers. The reflectance spectra generated atthe start of the CMP process comprises light reflected predominatelyfrom the barrier material. All of the following reflectance spectradiscussed hereinbelow will be represented similarly.

FIG. 5 is a reflectance spectra 500 of the barrier layer (Ta or TaN)being cleared away exposing a dielectric layer such as silicon dioxide.Continued polishing after the barrier layer has been removed can produceproblems such as the dishing effect described hereinabove that occurswhen other materials are exposed and removed at a faster rate than thedielectric layer. Another problem is too much dielectric removal. Asdescribed in FIG. 4, three probes were used to take measurements, eachprobe measured different areas of the wafer. Note that reflectancespectra 500 of the dielectric layer have a characteristic very similarto the barrier layer. Even though reflectance spectra 400 of FIG. 4 andreflectance spectra 500 of FIG. 5 look similar, reflectance spectra 500of FIG. 5 comprises light reflected predominately from the dielectricmaterial. As shown, the peaking of reflectance spectra 500 occurs inalmost an identical location at slightly less than 500 nanometers.Reliably detecting the difference between the two reflectance spectrahas proven less than adequate for this situation. The result is poor endpoint detection that produces a wide process variance including bothunder and over polishing.

FIG. 6 is an intermediate reflectance spectra 600 that occurs as abarrier layer is thinned in accordance with the present invention. Inparticular, the layer being removed in FIG. 6 comprises Ta or TaN.Intermediate reflectance spectra 600 is substantially different from thecase of reflected light that is predominately from the barrier materialas shown in FIG. 4 or the case of reflected light that is predominatelyfrom the underlying dielectric layer as shown in FIG. 5. Of note is thedip in magnitude that occurs between a wavelengths of 550 to 600nanometers. It is believed that the characteristic intermediatereflectance spectra 600 occurs for refractory metals and alloys thereofsuch as Ti, Cr, Mo, W, Rh, Ru, Re, WSi, and WNxCy although all have notbeen tested at this time. It is also possible for this characteristicintermediate reflectance spectra to occur for other materials with equalusefulness to a semiconductor manufacturing process.

The theory behind this discovery is that the material as it is thinnedenters a transparent phase to the measurement light or light pulse. Inother words, the reflected light is modified by both the surfacematerial (barrier material) upon which the light impinges and theunderlying layer (dielectric material). A beneficial factor in usingintermediate reflectance spectra 600 is that it occurs before all thesurface material is removed and that it is substantially different fromthe preceding and following reflectance spectra measured during thechemical mechanical planarization process. For example, a layer of Ta orTaN deposited to form a barrier layer in a copper interconnect isdeposited having a thickness of approximately 500 angstroms. Performinga chemical mechanical planarization process to remove the barrier layeron the surface of the wafer has found intermediate reflectance spectra600 to occur when the barrier layer has been thinned to approximately100 angstroms. The average thickness when this transparent phase occursand the detectable range around the median should be characterized foreach process and material. In this example, intermediate reflectancespectra 600 is highly desirable because it occurs immediately before theall the material is removed or cleared. Thus, allowing a clear signal tobe propagated to the CMP system that the clearing phase will beapproaching and to begin looking for the characteristic reflectancespectra corresponding to the material being cleared away such that theunderlying layer is exposed. This provides increased control indetermining the end point of the process thereby preventing under orover polishing and providing a process control that is uniform fromwafer to wafer and lot to lot.

Normalization of the reflectance spectra measured from the wafer is atechnique that helps to distinguish the three different reflectancespectra. In one embodiment, the in situ measured reflectance spectra isnormalized against a known (reference) reflectance spectra where thereflected light is predominately from the surface material beingremoved. Ideally, the known reflectance spectra used for normalizationis taken from the wafer during the chemical mechanical planarizationprocess at the beginning of the material removal process. FIG. 7 is anormalized reflectance spectra 700 for the case when the measurementproduces a reflectance spectra that is predominately from lightreflected from the targeted material being removed during the CMPprocess. As expected, normalized reflectance spectra 700 appear as analmost horizontal line since the measured reflectance spectra should bevery similar to the reference reflectance spectra used fornormalization. Normalized reflectance spectra 700 are shown having awavelength in a range from 400 to 800 nanometers.

FIG. 8 is a normalized reflectance spectra 800 for the case when thebarrier layer is thinned during the CMP process and the underlying layerof material modifies the reflected light. Note the dramatic differencewhen compared to normalized reflectance spectra 700 shown in FIG. 7. Theintermediate reflectance spectra when normalized take a characteristicsinusoidal shape. The sinusoidal shape has two peaks and one minimumover the range shown. The detection of this change in the CMP process isvery clear and can be acted upon in a time frame for accuratelycontrolling an end point for the process.

FIG. 9 is a normalized reflectance spectra 900 for the case when thebarrier layer is removed and the reflected light is predominately fromthe underlying dielectric layer. In general, normalized reflectancespectra 900 taken for the dielectric layer appears as a horizontal linehaving no significant minimum or maximum Although normalized reflectancespectra 900 of FIG. 9 do differ from normalized reflectance spectra 700of FIG. 7, the changes are not dramatic or substantial as the changeshown in FIG. 8. Moreover, normalized reflectance spectra 800 of FIG. 8differs significantly from both the normalized reflectance spectra 700of FIG. 7 and normalized reflectance spectra 900 of FIG. 9. Anotherbenefit is that normalized reflectance spectra 800 are extremely usefulin determining an endpoint when a thin layer of material is depositedand requires removal. This is a situation that may occur in the futureas geometries continue to shrink. Also, normalized reflectance spectraare only one of many formats that can be used to distinguish the threedifferent measured reflectance spectra described hereinabove. Forexample, fast Fourier transform analysis is another method that could beused to review the constituents of the reflectance spectra data beinggathered during the CMP process.

An illustrative method for planarizing a front surface of a wafer 100will now be described with reference to FIGS. 2, 3 and 6. The wafer 100is placed in a carrier 301 (step 600) and transported adjacent apolishing pad 309. The carrier 301 holds the wafer 100 substantiallyparallel to the polishing pad 309 while the wafer 100 is pressed againstthe top surface of the polishing pad 309 (step 601). The carrier 301 maybe rotated or otherwise moved in relation to the polishing pad 309 toassist in uniformly removing material from the front surface of thewafer 100. The supporting surface 211 and attached polishing pad 309 mayalso be moved in relation to the front surface of the wafer 100 and ispreferably orbited. (step 602) The relative motion is necessary toremove material from the front surface of the wafer 100.

In an embodiment for the manufacture of copper interconnect a groove,trench, or via is etched into dielectric layer 201. Barrier layer 203 isdeposited on a surface of the semiconductor wafer such that the bottomand sidewalls of the grooves, trenches, or vias are covered. Copper 200is then deposited on the surface covering barrier layer 203 and fillingthe grooves, trenches, or vias. A first CMP process is deployed suchthat copper 200 on the surface of the wafer is removed. The first CMPprocess is stopped when barrier layer 203 is reached. Copper 200 remainsin the grooves, trenches, or vias level with barrier layer 203 on thesurface of the wafer. A second CMP process for removing barrier layer203 on the surface of the wafer follows.

An end-point system 308 with two or more probes 306 a-c may be used totake measurements across the front surface of the wafer 100 during theplanarization process. The end-point system 308 may reflect a lightsignal off the front surface of the wafer 100 using a flash lamp 317. Inan embodiment of the system, a broadband spectrum of light is usedhaving a wavelength ranging from 300 nanometers to 800 nanometers. Toprevent smearing, the light is pulsed for ten microseconds or less whichcorresponds to a wafer 100 traveling a small distance during themeasurement. In an embodiment of the system, the spot size of the lightpulse is made larger than the largest feature (containing the materialbeing removed) to remain on the wafer after the CMP process.

A spectrometer 318 may be used to convert the reflected light intoreflectance spectra data representing the intensity of the reflectedlight at a plurality of wavelengths. Linear encoders 320 and 321,computer modeling or other known methods for determining the physicallocation of the mechanical devices may be used to track the location ofthe carrier 301 and the supporting surface 211. This allows the locationon the front surface of the wafer 100 to be determined for eachmeasurement (step 603). A control system 311 may be used to analyze themeasurements from the spectrometer 318 and the location of themeasurements on the wafer 100 to determine the progress of theplanarization process and the condition of the wafer 100 (step 604).

The progress of the planarization process is monitored and analyzed(step 604) for the three different characteristic spectra correspondingto the conditions where the light reflected back is predominately frombarrier layer copper 200, the light reflected back is modified by theunderlying dielectric layer, and the light reflected is predominatelyfrom dielectric 201. In particular, the intermediate reflectance spectra(light reflected back is modified by the underlying dielectric layer)are sought after which indicates the CMP process is nearing a clearingstage. Normalizing the reflectance spectra of the measurements takenduring the CMP process would identify the three different characteristicreflectance spectra such as the sinusoidal shape of the normalizedintermediate reflectance spectra.

The control system 311 may also be used to determine if an increased ordecreased removal rate over a portion of the front surface of the wafer100 is desirable (step 606). If the wafer 100 is being uniformlyplanarized, further measurements may be taken and analyzed (back to step603). However, if the control system 311 determines the removal rateshould be increased or decreased in particular areas, one or moreplanarization process parameters may be altered. For example, thedown-force, slurry delivery profile, rotation speed of carrier, etc. maybe adjusted to improve the planarization process. If a multizone carrier301 is being used, an increased or decreased pressure may be exerted onthe back surface of the wafer 100 opposite areas on the front surface ofthe wafer 100 that required an increased or decreased removal raterespectively (step 607). After altering the planarization process,further measurements may be taken and analyzed (back to step 603) withappropriate steps as described above taken.

In an embodiment of the process, the end point detection step 605 beginswhen the intermediate reflectance spectra is detected. Once again, thisoccurs when the material being removed is thinned sufficiently where theunderlying material modifies the reflected light. For example, thenormalized reflectance data changes from a horizontal line to asinusoidal shape. When this point is detected it will be known that theremaining material is within a specific range of thickness on thesurface. The rate of material removal can also be calculated based onthe initial thickness of the deposited material and the time it hastaken to reach the intermediate reflectance spectra point. Severaldifferent steps can complete the CMP process. First, the CMP process canbe completed after a predetermined time period after the intermediatereflectance spectra has been identified. The predetermined time isselected to ensure removal of a l l the remaining material. Second, theCMP process is continued with or without modifications to the CCMprocess in anticipation of receiving the reflectance spectra indicatingthat all the material has been removed. Upon receiving the reflectancespectra indicating clearing of the material, the endpoint has beendetected and the CMP process is stopped. Finally, an overpolish step canbe added after the reflectance spectra is detected indicating thematerial is cleared to ensure all the material is removed. In all of thecases, the end point detection using the intermediate step allows finercontrol of the CMP process to prevent under polishing and overpolishing. If the control system 311 determines the wafer 100 has beensufficiently planarized (step 605) the wafer may be unloaded from thecarrier (step 608) and removed from the CMP tool. The planarizationresults for this wafer 100 may be used to determine an improvedplanarization process for incoming wafers (step 609). This allows animproved down-force, slurry delivery profile, rotation speed of carrier,pressure within multizone carrier, relative motions between wafer 100and polishing pad 309, etc. to be altered during the planarizationprocess for incoming wafers to further improve the planarizationprocess.

While at least one exemplary embodiment has been presented in theforegoing detailed description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the foregoing detailed description willprovide those skilled in the art with a convenient road map forimplementing the exemplary embodiment or exemplary embodiments. Itshould be understood that various changes can be made in the functionand arrangement of elements without departing from the scope of theinvention as set forth in the appended claims and the legal equivalentsthereof.

1. A method for detecting an endpoint of a chemical mechanical planarization (CMP) process comprising the steps of: providing a light pulse on an area of a surface of a semiconductor wafer; receiving light reflected from said area of said surface; analyzing a reflectance spectra; and repeating said steps listed hereinabove until an intermediate reflectance spectra is identified that has a sinusoidal shape when normalized.
 2. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 1 further including the steps of: identifying a change in said reflectance spectra corresponding to a layer of material being removed from said surface by the chemical mechanical planarization process and an underlying layer of a different material is exposed; and stopping the chemical mechanical planarization process.
 3. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 2 wherein said step of identifying a change in said reflectance spectra corresponding to a layer of material being removed from said surface by the chemical mechanical planarization process and an underlying layer of a different material is exposed further includes a step of overpolishing for a predetermined time period to ensure said layer of material is removed.
 4. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim I further includes the steps of: continuing the chemical mechanical planarization process for a predetermined time period; and stopping the chemical mechanical planarization process after said predetermined time period.
 5. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 1 wherein said step of providing a light pulse on an area of a surface of a semiconductor wafer further includes using a broadband spectrum of light such that an intensity of said reflected light is analyzed over a plurality of wavelengths.
 6. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 5 wherein said step of using a broadband spectrum of light such that an intensity of said reflected light is analyzed over a plurality of wavelengths further includes a step of providing light in a range of 300 to 800 nanometers in wavelength.
 7. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 1 wherein in said step of analyzing a reflectance spectra further includes a step of performing a fast fourier transform analysis on said reflectance spectra.
 8. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 1 wherein said step of repeating said steps listed hereinabove until an intermediate reflectance spectra is identified that has a sinusoidal shape when normalized further includes the steps of: varying a location of said light pulse on said surface of said wafer; and taking a diversity of reflectance spectra over time such that an entire surface of said semiconductor wafer is represented by said measurements in determining material uniformity, thickness, and removal rate.
 9. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 1 wherein said step of providing a light pulse on an area of a surface of a semiconductor wafer further includes a step of providing said light pulse for a time period of approximately ten microseconds or less.
 10. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 1 wherein said step of providing a light pulse on an area of a surface of a semiconductor wafer further includes a step of providing said light pulse having a spot size larger than a largest feature size to remain on said semiconductor wafer after the CMP process.
 11. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 1 wherein said step of providing a light pulse on an area of a surface of a semiconductor wafer further includes a step of using more than one probe to pulse and receive light.
 12. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 11 wherein said step of using more than one probe to pulse and receive light further includes the steps of: using more than one probe to pulse and receive light such that each probe measures a concentric band on said surface and said concentric bands measured by each probe combine to represent an entire surface of said semiconductor wafer; and overlapping measurements of each probe to an adjacent concentric band.
 13. A method for detecting an endpoint of a chemical mechanical planarization (CMP) process comprising the steps of: taking reflectance spectra data periodically on different areas of a surface of a semiconductor wafer during the CMP process; identifying a first reflectance spectra corresponding to a first layer of material on a surface of a semiconductor wafer such that said first reflectance spectra comprises light reflected predominately from said first layer of material; identifying a second reflectance spectra corresponding to said first layer of material on said surface being thinned such that said second reflectance spectra is modified by a second layer of material underlying said first layer of material; and identifying a third reflectance spectra corresponding to said first layer of material on said surface being substantially removed such that said third reflectance spectra comprises light reflected predominately from said second layer of material.
 14. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 13 wherein said step of taking reflectance spectra data periodically on different areas of a surface of a semiconductor wafer during the CMP process further includes a step of using a broadband spectrum of light ranging from 300 to 800 nanometers in wavelength to generate said reflectance spectra data.
 15. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 13 further including the steps of: normalizing said reflectance spectra data to said first reflectance spectra; and identifying when said normalized reflectance spectra data changes from an approximately linear shape to an approximately sinusoidal shape that corresponds to said second reflectance spectra.
 16. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 15 further including the steps of: continuing the CMP process for a predetermined time period; and ending the CMP process after said predetermined time period.
 17. The method for detecting an endpoint of a chemical mechanical planarization (CMP) process as recited in claim 15 further including the steps of: identifying when said normalized reflectance spectra data changes from said approximately sinusoidal shape to an approximately linear shape that corresponds to said third reflectance spectra data; overpolishing for a predetermined time period; and ending the CMP process after said predetermined time period.
 18. A method of wafer processing including end point detection for a chemical mechanical planarization process (CMP) comprising the steps of: forming at least one trench in a dielectric layer; depositing a barrier material on a surface of a semiconductor wafer such that said barrier material forms a layer on a bottom and sidewalls of said at least one trench; depositing copper on said surface of the semiconductor wafer such that said at least one trench is filled with copper; performing a first CMP process to remove a layer of copper on said surface of the semiconductor wafer such that said copper remains in said at least one trench; initiating a second CMP process to remove said layer of barrier material on said surface of the semiconductor wafer; taking reflectance spectra data on different areas of said surface of said semiconductor wafer using a broadband spectrum of light ranging from 300 nanometers to 800 nanometers in wavelength; identifying when said barrier metal has been thinned such that said reflectance spectra data is modified by said dielectric layer underlying said barrier layer; and continuing with said second CMP process knowing an approximate thickness of said barrier layer that remains.
 19. The method of manufacturing as recited in claim 18 further including a step of: identifying when said reflectance spectra data corresponds to reflected light predominately from said dielectric layer; and overpolishing to ensure complete removal of said barrier material on said surface of the semiconductor wafer.
 20. The method of manufacturing as recited in claim 18 wherein said step of depositing a barrier metal on a surface of a semiconductor wafer such that said barrier metal forms a layer on a bottom and sidewalls of said at least one trench further includes a step of depositing tantalum as said barrier material.
 21. The method of manufacturing as recited in claim 18 wherein said step of depositing a barrier metal on a surface of a semiconductor wafer such that said barrier metal forms a layer on a bottom and side walls of said at least one trench further includes a step of depositing tantalum nitride as said barrier material. 